In the wake of the ongoing deregulations of the electric power markets, load transmission and wheeling of power from distant generators to local consumers has become common practice. As a consequence of the competition between utilities and the emerging need to optimize assets, substantially increased amounts of power are transmitted through the existing networks, invariably causing congestion, transmission bottlenecks and/or oscillations of parts of the power transmission systems. In this regard, electrical transmission networks are highly dynamic, and in response to changing network states, loads or power injected by generating units, the power flow over alternate transmissions paths may need to be redistributed. Such adjustments are made according to the current topology and electrical flow situation of the electrical transmission network, preferably by means of so-called Power Flow Control Devices (PFD). These devices are installed at transmission line stations to adjust power flow in each transmission line, so that power can be guided to flow in a safe, stable and balanced manner in a large number of lines within the electrical transmission network. PFDs in general may comprise mechanical components and their set-points are updated on a time-scale of hours.
By way of example, the unpublished International Patent Application PCT/CH 2005/000125 is concerned with the challenges of power flow management in electrical transmission networks caused by the integration of power systems across several regions with differing price and/or demand levels. PFDs such as a series capacitor, a Phase Angle Regulator, a Phase Shifting Transformer (PST) or a Flexible Alternating Current Transmission System (FACTS) device improve dynamic performance of electrical transmission networks. They are designed to supply reactive power to support voltage and provide stability enhancements, thereby allowing transmission facilities to be loaded to levels approaching their ultimate thermal capacity.
Fast network controllers or Power Flow Control Devices (PFD) which are based on power electronic semiconductor components and which are devoid of mechanical switches enable response times in the millisecond range. They include, among others, the aforementioned Flexible Alternating Current Transmission System (FACTS) devices as well as High Voltage DC (HVDC) devices. HVDC devices comprise power converters or voltage source converters for generating or consuming active power, which converters are based on a multitude of semiconductor components or modules that are individually controlled by control signals produced by gate drives or other control hardware of a converter control unit.
In general, electromechanical oscillations in electric power transmission networks interconnecting several alternating current generators have a frequency of less than a few Hz and are considered acceptable as long as they decay. They are initiated by the normal small changes in the system load, and they are a characteristic of any power system. Frequent but small oscillations may lead to wear and tear of power plant equipment especially governor servo equipment. Insufficiently damped oscillations may occur when the operating point of the power system is changed, e.g. due to a new distribution of power flows following a connection or disconnection of generators, loads and/or transmission lines. Likewise, the interconnection of several existing power grids, even if the latter do not individually present any badly damped oscillations prior to their interconnection, may give rise to insufficiently damped oscillations. In these cases, an increase in the transmitted power of a few MW may make the difference between stable oscillations and unstable oscillations which have the potential to cause a system collapse or result in lost of synchronism, lost of interconnections and ultimately the inability to supply electric power to the customer. Constant monitoring of the power system can help a network operator to accurately assess power system states and avoid a total blackout by taking appropriate actions such as the connection of specially designed damping equipment.
In the Patent Application EP-A 1 489 714, a system quantity or signal such as e.g. the amplitude or angle of the voltage or current at a selected node of the network is sampled, and the parameters of a parametric model representing the behaviour of a power transmission system, or a particular aspect thereof, are estimated. This process is carried out in an adaptive manner, i.e. every time a new value of the system quantity is measured, the parameters of the model are updated recursively. Finally, from the estimated parameters of the model, the parameters of the oscillatory modes are computed, and their oscillation frequency and damping properties are quantified and presented to the operator. This process enables an almost instantaneous analysis of the oscillation state of the power system as compared to a non-adaptive identification process relying on the analysis of sampled data collected over a time-window of several minutes and evaluated only at the end of this time-window.
In the article by M. Larsson et al. “Improvement of Cross-border Trading Capabilities through Wide-area Control of FACTS”, Proceedings of Bulk Power System Dynamics and Control VI, 22-27 August, Cortina D'Ampezzo, Italy, 2004, coordination of a multitude of FACTS devices is proposed. A secondary control loop generates the set-points for the primary FACTS controllers, based on global or wide-area information. The latter comprises state snapshots from a wide-area measurement system including a relatively large number of Phasor Measurement Units (PMUs). The subsequent mathematical optimization of the FACTS set-points occurs in real time with respect to loadability criteria, voltage security assessments and/or accurate stability margins.
The article by E. Lerch et al. “Advanced SVC control for damping power system oscillations”, IEEE Transactions on Power Systems, Vol. 6, No. 2, May 1991, pages: 524-535, ISSN: 0885-8950, proposes to improve Static Var Compensation (SVC) control by introducing signals which reflect power system oscillations, using phase angle signals estimated from measurements of local state variables (voltage and power) at the location of the Static Var Compensator itself. Maximum damping is achieved by employing bang-bang control and additional filters for eliminating interference signals from the electromechanical oscillation signal having an oscillation frequency known in advance.
The article by S. G. Johansson et al. “Power System Stability Benefit With Vsc Dc-Transmission Systems” in Cigre General session 2004, Paper B4-204, Paris; discusses different aspects of the controllability of a Voltage Source Converter (VSC) HVDC transmission. In particular, the freedom of active and reactive power control for this type of device is illustrated, as well as its impact on the transmission system.